For unprocessed well stream in ordinary subsea pipelines, the temperature of oil, gas and produced water will drop rapidly due to cooling from the surrounding seawater. The low temperature results in undesired fluid properties. At high pressures hydrates start to precipitate already at temperatures in the range of 20-25° C. Large amounts of hydrate, which is similar to ice crystals, can precipitate on the pipe wall and cause blocking of well stream transport. For some fields wax formation in the flowing crude may also cause operational problems due to increased pressure loss in the pipeline. The viscosity of waxy oil can be of such magnitude, that full “shut in wellhead pressure” will not be sufficient for getting the cold fluid on stream again after long shut downs. The use of chemicals to remove hydrates will in practice mean to use methanol or glycol. The disadvantage with use of chemicals is that large amounts are often needed and implies a risk to the environment if leakage should occur.
A way to remove hydrates is to supply heat to the pipe content. Direct Electrical Heating (DEH) has been developed and qualified for heating of pipelines and is installed on several subsea pipelines in the North Sea. Electrical heating of pipelines implies reduced investments of depressurizing systems and recovery plants for chemical residual products. Especially for deep-water fields electrical heating of pipelines is attractive for achieving reliable operation of transport pipelines. The method, which has used a single phase 50/60 Hz powered system, is presented in [1, 2].
Both single and multiple pipelines are installed according to the principle described in [1, 2]. For dual pipelines the supply cables, dynamic riser and static feeder, are now designed with four conductors. Due to the high currents required for feeding the DEH system, large conductor cross sections are needed to avoid over-heating of these cables.
The DEH system is fed from an AC power supply by special supply cables (riser cables, static single core feeding cables etc) to the pipeline connection point at the near end and to the piggyback cable, which is routed along the pipeline to the pipe connection at the far end. At both connection ends the pipeline is supplied with anodes. These anodes provide grounding of the pipeline and transfer some of the current to seawater. In order to keep the transfer current density both for the anodes and for pipe steel through possible cracks in the pipe thermal coating, a sufficient number of anodes distributed over approximately 50 m length (current transfer zone, CTZ) are required. The return current flows partly through seawater and partly through the pipeline, which is intended to be heated. The circuits are not closed loop circuits, due among others to safety reasons as the anodes and pipelines are not electrically isolated from the surrounding seawater. In addition to the anodes at each end, the pipeline may be supplied with distributed anodes in order to limit the pipe voltage, which occur if the magnetic and/or electrical properties of the pipe steel (in the individual pipe joints) varies along the pipeline.
In addition to the problem related to high cross sections of the riser cables, the prior art methods of DEH includes problems with AC corrosion of anodes. It is also problems related to safety distances to steel structures in particular close to CTZ (Current Transfer Zones) of traditional DEH systems for subsea pipelines.
It is known from US2010101663 (A1) a fluid flow within a transportation pipeline that is heated with low voltage, high current electrical energy induced into a conductive closed loop structure by one or more transformers. The closed loop structure is preferably a fluid transportation pipeline constructed of electrically conductive sections of pipeline. The amount of current induced is sufficient in relation to the inherent resistivity of the conductive sections to cause the generation of heat within the pipeline sections. By conductive and convective heat transfer, the heat induced into the pipeline structure is transferred to a fluid flow within the pipeline. The current is preferably an alternating current of frequency which causes a majority of the current to travel at or near the outer surfaces of the pipeline sections which increases the effective resistivity of the sections and heat generation therein.
WO2007011230 (A1) discloses a power system that provides electrical power to an electric load circuit comprising a three-phase electrical power generation and power transmission system 1 being coupled to an electric load 4,2,21. The three-phase generation and transmission system is connected to said subsea located electric load 4,2,21 via a three-phase to two-phase transformer 2, said electric load being connected to the secondary side of said three-phase to two-phase transformer 2 so as to form a balanced electric load on the three-phase electrical power generation and power transmission system 1. The power system may further be connected to an end load 30 for providing power to components or equipment connected to or powered by the end load 30.
EP2166637 (A1) discloses a power supply arrangement for supplying electrical power to a pipeline. The power supply arrangement is a direct electrical heating system for a pipeline system that comprises three phase transformer (2), a symmetrisation unit (14) and a compensation unit (22).
Other examples of systems and methods for heating pipelines can be found in US 2003/0016028 A1, U.S. Pat. No. 6,509,557 B1 and NO 304533 B1.
The present invention discloses new configurations of the electrical circuits applicable for both single and multiple pipelines, which implies that the number of single core supply cables and the conductor cross section can be reduced. The power losses in the cables are significantly reduced, which solve a problem with overheating. This makes improvement especially for the supply cables, multi core riser cable, routed from topside to the connection of the DEH cables subsea. The new method implies that both number of single core cables and cable conductor cross section can be reduced and hence reduces investment cost considerably.
Furthermore it is an object according to the present invention to overcome or reduce problems related to AC corrosion and exposure of magnetic field and stray current in neighbouring structures as indicated above.